1. Field of the Invention
The present invention relates to a display device having a video bandwidth controller and a method for controlling the video bandwidth. More particularly, the invention relates to a display device having a video bandwidth controller and to a method for displaying two or more different types of red (R), green (G) and blue (B) video signals.
2. Description of the Related Art
Conventionally, display devices having a resolution equal to or higher than that of RGB video signals being input to the display device have been used for displaying the RGB video signals. It is known to switch from a RGB video signals with a normal resolution to RGB video signals with a higher resolution. It is also known that RGB signals with different resolutions can be generated in a single computer. A display device which is connected to a computer having such a function needs to display RGB signals with different resolutions, as disclosed in "MAC LIFE No. 53, January 1993".
In such a case, conventionally, the resolution of a cathode-ray tube (CRT) is fixed. Furthermore, in a stripe type CRT as disclosed in "NHK Television Technology Textbook (Vol. 1)", electron beams which pass through a shadow mask actually act on the display area of the CRT. Referring to FIGS. 1 and 2, electron beams from three electron guns 14 which are respectively used for red (R) beam, green (G) beam and blue (B) beam pass through a slit 12 in the shadow mask 11 and make RGB phosphors 13 emit light. In a CRT configured in this manner, pictures are represented by 30 percent or less of the entire electron beam emitted from the electron guns.
In this case, the RGB signals being provided to the display device are produced based on a dot clock which is a reference clock corresponding to one dot. Therefore, the RGB is signals level may change on a per dot basis. For example, the relationship among an input signal S0, a beam B0 which passes through a slit 12 of the shadow mask 11, and light emission P0 of a phosphor surface, in conjunction with the positional relationship between the slit 12 and the phosphor surface, is shown in FIGS. 3A to 3E. Referring to FIGS. 3A to 3E, the input signal is a voltage (E) signal, and the beams emitted from electron guns according to the input signal are scanned in such a manner that the position of the beams with respect to the shadow mask 11 sequentially moves as time (t) elapses. When the input signal S0 is input, beam B0 which passes through the slit 12 hits the phosphor 13 on the CRT.
Considering the frequency characteristic of an input signal, however, the actual input signal would be an input signal S1 as shown in FIG. 4C. Comparing the original input signal S0 in FIG. 3C, with the input signal S1 in FIG. 4A, the shadow mask pitch of the CRT is smaller than the pitch of the RGB signal corresponding to one dot, therefore, there would be cases in which the width of the black portion corresponding to one dot displayed on the CRT is smaller than that of the original signal. This phenomenon causes the line thickness of a black character displayed on white background to be partially reduced.
Furthermore, for an input signal S2, which has a still lower frequency characteristic, a passing beam B2, phosphor surface light emission luminance P2, and phosphor surface luminance L2 would be as illustrated in FIGS. 5A to 5E. As mentioned above, the input signals S1 and S2 are input to the display device as the RGB signals, the line thickness of a black character displayed on white background is partially reduced and the character's appearance becomes blurred.